This invention relates to methods and apparatus for volumetric computed tomograph (CT) reconstruction and more specifically to methods and apparatus for fan parallel cone beam volumetric CT reconstruction utilizing a flat panel detector.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Three-dimensional reconstruction of scanned CT images is possible. A direct fan parallel reconstruction method is used for data acquired using a flat panel detector. The use of parallel reconstruction advantageously provides an amount of aliasing cancellation. However, in at least one known method, a radial interpolation step is required when rebinning from fan beam to parallel data. This interpolation step produces a loss of resolution.
It would therefore be desirable to provide methods and apparatus for direct fan parallel, three dimensional reconstruction of computed tomographic images that avoid the loss of resolution associated with radial interpolation and that retain the image quality and backprojection speed gains associated with parallel reconstruction.